I personally think that cars and modular cars are kind of bad in rust.
Because i belive people will spam all the ground with pretty much everything they have here and there, to prevent people to drive.
It will be hard to drive a car.
Maybee crossbikes will work out. Maybee spiderbots
But in my opinion rust is more of a game for hobo helicopters.
You can cross over parts of the map, you can struggle with oil.
It can leave a big trail of smoke behind, you can snipe these helicopter people, the helicopter itself.
You possible can buff pistols and make players be able to use these from the helicopter.
I really belive that the hobo style fits well with helicopters.
The next way to transport in my opinion is making ship foundations or modular ships that you can upgrade or design, in a similiar way like you design bases.
Upgrade and place canons on it and going to the next step, crossing water to find other islands, with hobo helicopters and ships. Making epic battles, find dead ships and loot these, going to dive for them.
I belive that the air and the water are the better ways to create a transport and crossing option, since it is naturally harder to mill the air and the water with buildings.